CN111936648B - High-strength galvanized steel sheet, high-strength member, and method for producing same - Google Patents

High-strength galvanized steel sheet, high-strength member, and method for producing same Download PDF

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CN111936648B
CN111936648B CN201980023153.9A CN201980023153A CN111936648B CN 111936648 B CN111936648 B CN 111936648B CN 201980023153 A CN201980023153 A CN 201980023153A CN 111936648 B CN111936648 B CN 111936648B
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steel sheet
less
strength
plating
steel
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CN111936648A (en
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吉富裕美
中垣内达也
木庭正贵
铃木善继
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JFE Steel Corp
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JFE Steel Corp
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel

Abstract

The invention provides a high-strength galvanized steel sheet with excellent plating property and bending property, a high-strength component and a manufacturing method thereof. The high-strength galvanized steel sheet of the present invention comprises a steel sheet and a galvanized layer on the surface of the steel sheet, wherein the steel sheet has a composition containing predetermined component elements, and a steel structure in which the average particle diameter of at least 1 type of inclusion containing Al, Si, Mg and Ca present in the range from the surface to the 1/3 th position is 50 [ mu ] m or less, the average closest distance of the inclusion is 20 [ mu ] m or more, and the plating adhesion amount per surface of the galvanized layer is 20g/m2～120g/m2The amount of diffusible hydrogen contained in the steel is less than 0.25 mass ppm, and the tensile strength is 1100MPa or more.

Description

High-strength galvanized steel sheet, high-strength member, and method for producing same
Technical Field
The present invention relates to a high-strength galvanized steel sheet having excellent plating properties and bendability and suitable for building materials and collision-resistant parts for automobiles, a high-strength part, and a method for producing the same.
Background
Recently, there is a strong demand for safety against collision and fuel efficiency improvement in automobiles, and steel sheets as component materials are increasingly strengthened. Further, automobiles are widely spread in the global area, and are used in various applications in various regions and climates, and therefore, high rust resistance is required for steel sheets as component materials.
Generally, if the strength of a steel sheet is increased, the workability is lowered. In particular, the plated steel sheet tends to have inferior workability as compared with a steel sheet not plated
Further, it is known that if a large amount of alloying elements are contained for the purpose of increasing the strength, it is difficult to form a good-quality plating film on a steel sheet. In addition, if plating of Zn, Ni, or the like is performed, hydrogen that intrudes during the manufacturing process is difficult to be released from the steel.
In addition, development of a steel sheet having excellent bendability has been carried out. From the characteristics of the processing method, how to design the most severe processing conditions at the time of bending, that is, the stress concentration portion is disclosed as a solution to the problem. In particular, in the case of a steel sheet having 2 or more steel structures with different hardness, the deformation is concentrated at the interface of the steel structures, and a defect of a fine hole is easily formed, resulting in deterioration of bendability.
In addition, in order to adhere to a good plating, a solution is also taken in which the furnace atmosphere in the annealing/plating step is controlled.
In non-patent documents 1 and 2, the steel structure of the steel sheet is made of ferrite and martensite, but once the steel structure of ferrite and martensite is formed, the steel is tempered to soften the martensite and improve the bendability.
Patent document 1 discloses a high-strength steel sheet having a maximum tensile stress of 900MPa or more, which is a structure homogeneity index indicating the homogeneity of the steel sheet, which is an index indicating the homogeneity of the steel sheet given by the standard deviation of rockwell hardness of the surface of the steel sheet, and which is good in ductility and bendability, and a method for manufacturing the same. In a method for obtaining a result of improving inhomogeneity of a solidification structure at the time of casting, which is a factor affecting bendability, a steel sheet having a maximum tensile stress of 900MPa or more and excellent bendability is proposed by the method.
In addition, in patent document 1, in order to ensure good plating properties, in this case, a hydrogen concentration of 1 to 60 vol% is formed in an annealing furnace for continuous hot dip galvanizing of wire, and the remainder is made of N2、H2O、O2And unavoidable impurities, and the log (P) of the partial pressure of water and the partial pressure of hydrogen in the atmosphereH2O/PH2) Specified as-3. ltoreq. log (P)H2O/PH2)≤-0.5。
In patent document 2, in a steel sheet having a composite structure containing 50% or more of bainite and 3 to 30% of retained austenite, the ratio of the hardness Hvs of the surface layer of the steel sheet to the hardness Hvb of the steel sheet in the thickness of 1/4 is defined to be 0.35 to 0.90. Further, the plating property is secured in a high alloy system by annealing in an atmosphere having a log (water partial pressure/hydrogen partial pressure) of-3.0 to 0.0.
Patent document 3 discloses a method for producing a plated steel sheet, in which bendability is ensured by defining a decarburized ferrite layer, and an atmosphere is adjusted to an atmosphere consisting of 2 to 20 vol% of hydrogen and the remainder containing nitrogen and impurities and having a dew point of more than-30 ℃ and 20 ℃ or less.
Documents of the prior art
Patent document
Patent document 1: japanese patent laid-open publication No. 2011-
Patent document 2: japanese patent laid-open publication No. 2013-163827
Patent document 3: japanese patent laid-open publication No. 2017-048412
Non-patent document
Non-patent document 1: changchuanhao flat, another 5 people, "influence of metallic structure on bending workability of 980MPa grade ultra-high strength steel sheet", CAMP-ISIJ, vol.20(2007), p.437, issued by japan iron and steel association
Non-patent document 2: mitsunor, another 3, "influence of texture on stretch flange formability of ultra-high strength cold rolled steel sheets", CAMP-ISIJ, vol.13(2000), p.391, issued by japan iron and steel association
Disclosure of Invention
In order to improve the bendability of steel sheets, the steel structure has been optimized, but only a certain degree of improvement is required, and further improvement is required. In addition, when plating is performed on a high alloy steel sheet, it is considered that hydrogen in the atmosphere in the plating step becomes hydrogen in the steel remaining in the steel sheet product. It is considered that hydrogen in the steel hinders improvement of bendability. In addition, improvement of bendability and plating property are also required at the same time.
The present invention aims to provide a high-strength galvanized steel sheet excellent in plating property and bendability, a high-strength member, and a method for producing the same.
The term "high strength" as used herein means a Tensile Strength (TS) of 1100MP or more.
The present inventors have conducted intensive studies to solve the above problems. As a result, it has been found that a high-strength galvanized steel sheet having excellent bendability and plating properties can be obtained by adjusting the steel sheet to have a specific composition, adjusting the inclusion content in the range from the surface to the 1/3 th position to have a steel structure in a predetermined existing state, and appropriately adjusting the amount of hydrogen remaining in the steel. The high-strength galvanized steel sheet according to the present invention can be produced by appropriately adjusting the conditions of the respective production steps, such as the conditions of the furnace atmosphere during annealing. Specifically, the present invention provides the following.
[1] A high-strength galvanized steel sheet comprising a steel sheet and a galvanized layer formed on the surface of the steel sheet,
the steel sheet has the following composition and steel structure,
the above composition is a steel composition containing, in mass%, C: 0.08-0.20%, Si: less than 2.0%, Mn: 1.5% -3.5%, P: 0.02% or less, S: 0.002% or less, Al: 0.10% or less and N: less than 0.006%, the remainder being Fe and unavoidable impurities,
in the steel structure, the average grain size of at least 1 type of inclusion containing Al, Si, Mg and Ca existing in the range from the surface to the 1/3 th position of the plate thickness is 50 μm or less, the average closest distance of the inclusions is 20 μm or more,
the plating adhesion per one side of the above zinc plating layer was 20g/m2～120g/m2，
The amount of diffusible hydrogen contained in the steel is less than 0.25 mass ppm,
the tensile strength of the high-strength galvanized steel sheet is 1100MPa or more.
[2] The high-strength galvanized steel sheet according to [1], wherein a mass ratio of the Si content to the Mn content (Si/Mn) in the steel is less than 0.1.
[3] The high-strength galvanized steel sheet according to [1] or [2], wherein the above-mentioned composition further contains at least 1 of the following (1) to (3) in mass%.
(1) 1 or more of Ti, Nb, V and Zr: 0.005 to 0.1 percent in total
(2) 1 or more of Mo, Cr, Cu and Ni: 0.01 to 0.5 percent of the total
(3)B：0.0003％～0.005％
[4] The high-strength galvanized steel sheet according to any one of [1] to [3], wherein the composition further contains, in mass%, Sb: 0.001% -0.1% and Sn: 0.001-0.1% of at least 1.
[5] The high-strength galvanized steel sheet according to any one of [1] to [4], wherein the composition further contains, in mass%, Ca: less than 0.0005%.
[6] The high-strength galvanized steel sheet according to any one of [1] to [5], wherein the steel structure has, in terms of area percentage, 30% to 85% of martensite, 60% or less (including 0%) of ferrite, 15% or less (including 0%) of bainite, and less than 5% or less (including 0%) of retained austenite,
the ferrite has an average grain diameter of 15 μm or less.
[7] A method for producing a high-strength galvanized steel sheet, comprising the steps of:
a casting step of casting a steel having the composition described in any one of [1] to [5] under a condition that a molten steel flow velocity at a solidification interface in the vicinity of a meniscus of a mold is 16 cm/sec or more to produce a steel material;
a hot rolling step of hot rolling the steel slab after the casting step;
a pickling step of pickling the steel sheet after the hot rolling step;
a cold rolling step of cold rolling the steel sheet after the pickling step at a reduction ratio of 20% to 80%;
a pretreatment step of heating the steel sheet after the cold rolling step to a pretreatment heating temperature of 720 ℃ to 880 ℃, thereafter cooling the steel sheet from the pretreatment heating temperature to 500 ℃ at an average cooling rate of 2 ℃/sec or more, cooling the steel sheet from 499 ℃ to 200 ℃ at an average cooling rate of 3 ℃/sec or more, and then pickling the steel sheet;
an annealing step of heating the steel sheet after the pretreatment step at an annealing temperature of 740 ℃ to (Ac3+20) ° C and then cooling the steel sheet from the annealing temperature to 600 ℃ at an average cooling rate of 3 ℃/sec or more, using a continuous annealing line so that the hydrogen concentration in the furnace atmosphere in a temperature range of 500 ℃ or more is greater than 0 vol% and 12 vol% or less; and
and a plating step of performing plating treatment on the steel sheet after the annealing step, and cooling the steel sheet after the plating treatment at an average cooling rate of 3 ℃/sec or more in a temperature range of 450 to 250 ℃.
[8] The method for producing a high-strength galvanized steel sheet according to [7], wherein, in the pretreatment step, when heating is performed at the pretreatment heating temperature, a dew point of an atmosphere of 600 ℃ to the pretreatment heating temperature is Y ℃ or higher,
in the annealing step, when the heating is performed at the annealing temperature, the dew point of the continuous annealing line in a temperature region of 700 ℃ or higher is Z ℃ or lower, and the dew point is Z ℃ or lower
When the pretreatment heating temperature is X ℃,
the X, Y and Z satisfy the following relational expressions (i), (ii) or (iii).
(i) X is more than or equal to 720 and less than or equal to 800, and Y-Z is more than or equal to-5
(ii) X is more than 800 and less than or equal to 840, and Y-Z is more than or equal to 0
(iii) X is more than 840 and less than or equal to 880, and Y-Z is more than or equal to 5
[9] The method for producing a high-strength galvanized steel sheet according to item [7] or [8], further comprising a width trimming step of performing width trimming after the plating step.
[10] The method for producing a high-strength galvanized steel sheet according to any one of [7] to [9], further comprising a post-treatment step of heating in an atmosphere having a hydrogen concentration of 5 vol% or less and a dew point of 50 ℃ or less for 30 seconds or more in a temperature range of 50 to 400 ℃ after the annealing step or after the plating step.
[11] The method for producing a high-strength galvanized steel sheet according to any one of [7] to [10], wherein in the plating step, an alloying treatment is performed immediately after the plating treatment.
[12] A high-strength member obtained by at least one of forming and welding the high-strength galvanized steel sheet according to any one of [1] to [6 ].
[13] A method for manufacturing a high-strength member, comprising a step of subjecting a high-strength galvanized steel sheet manufactured by the method for manufacturing a high-strength galvanized steel sheet according to any one of [7] to [11] to at least one of forming and welding.
According to the present invention, a high-strength galvanized steel sheet excellent in plating property and bendability, a high-strength member, and a method for producing the same can be provided. When the high-strength galvanized steel sheet of the invention is used for a frame member of an automobile body, great contribution is made to improvement of collision safety and weight reduction.
Drawings
FIG. 1 is a graph showing an example of the relationship between the amount of diffusible hydrogen in steel and R/t.
Detailed Description
Hereinafter, embodiments of the present invention will be described. The present invention is not limited to the following embodiments.
The high-strength galvanized steel sheet of the present invention has a steel sheet and a galvanized layer formed on the surface of the steel sheet. First, the composition of the steel sheet (steel composition) will be described. In the description of the component composition of the steel sheet, "%" as a unit of component content means "% by mass".
C：0.08％～0.20％
C is an element effective for increasing the strength of the steel sheet, and contributes to increasing the strength by forming martensite, which is one of the hard phases of the steel structure. In addition, the formation of fine alloy compounds or alloy carbonitrides with carbide-forming elements such as Nb, Ti, V, and Zr also contributes to the enhancement of strength according to the production method. In order to obtain these effects, the C content needs to be 0.08% or more. On the other hand, if the C content exceeds 0.20%, the martensite becomes excessively hard, and the bending workability tends not to be improved even if the inclusions and the amount of hydrogen in the steel are controlled. Therefore, the content of C is 0.08-0.20%. From the viewpoint of stabilizing TS to 1100MPa or more, the C content is preferably 0.09% or more.
Si: less than 2.0 percent
Si is an element that contributes to increase in strength mainly by solid solution strengthening, and contributes to not only improvement in strength but also improvement in the balance between strength and ductility with less reduction in ductility compared to improvement in strength. The increase in ductility brings about an improvement in bendability. On the other hand, Si is likely to form Si-based oxides on the surface of the steel sheet, and may cause no plating, and if it is contained excessively, a significant scale is formed during hot rolling, leaving scale marks on the surface of the steel sheet, resulting in deterioration of surface properties and reduction in pickling properties. Therefore, the amount required for securing the strength may be added, and the Si content is made less than 2.0% from the viewpoint of the plating property. In addition, the Si content is preferably 0.5% or less, more preferably 0.3% or less, from the viewpoint of obtaining more excellent plating properties. The lower limit of the Si content is not particularly limited, and when the Si content is less than 0.001%, control during production tends to be difficult, and therefore the Si content is preferably 0.001% or more.
Mn：1.5％～3.5％
Mn is effective as an element contributing to high strength by solid-solution strengthening and martensite formation, and in order to obtain this effect, it is necessary to contain 1.5% or more. The Mn content is preferably 1.9% or more. On the other hand, if the Mn content exceeds 3.5%, unevenness of the steel structure is likely to occur due to segregation of Mn or the like, resulting in a decrease in workability, and Mn is likely to be externally oxidized as an oxide or a complex oxide on the surface of the steel sheet, thereby causing no plating in some cases. Therefore, the Mn content is set to 3.5% or less.
P: less than 0.02%
P is an effective element contributing to the increase in strength of the steel sheet by solid-solution strengthening, but has an influence on the plating property. In particular, deterioration of wettability with the steel sheet and delay of alloying rate of the coating layer are caused, and the influence on obtaining a high alloy system such as a high strength steel sheet is particularly large. Therefore, the P content is set to 0.02% or less. The P content is preferably 0.01% or less. The lower limit of the P content is not particularly specified, and when less than 0.0001%, it results in a decrease in production efficiency and an increase in dephosphorization cost in the production process, the P content is preferably 0.0001% or more.
S: less than 0.002%
S tends to form sulfide-based inclusions in steel. Particularly, when a large amount of Mn is added for high strength, MnS-based inclusions are easily formed. It is preferable to reduce the amount of S as much as possible because S causes not only a deterioration in bendability but also thermal brittleness and adversely affects the production process. Up to 0.002% is allowable in the present invention. The lower limit of the S content is not particularly specified, and when less than 0.0001%, a reduction in production efficiency and an increase in cost are caused in the production process, so the S content is preferably 0.0001% or more.
Al: less than 0.10%
Al is added as a deoxidizer. When Al is added as a deoxidizer, it is preferable to contain 0.001% or more of Al in order to obtain the effect. On the other hand, if the Al content exceeds 0.10%, inclusions are likely to be formed in the production process, and the bendability is deteriorated. Therefore, the Al content is set to 0.10% or less, preferably 0.08% or less in terms of sol.al in the steel.
N: less than 0.006%
If the N content exceeds 0.006%, excessive nitrides may be generated in the steel to lower workability and cause deterioration in the surface properties of the steel sheet. Therefore, the N content is set to 0.006% or less, preferably 0.005% or less. In the case where ferrite is present, the content is preferably extremely small from the viewpoint of improvement in ductility due to cleaning thereof, but since this leads to reduction in production efficiency and increase in cost in the production process, the N content is preferably 0.0001% or more.
The mass ratio of the Si content to the Mn content (Si/Mn) in the steel is less than 0.1
In order to obtain more excellent plating properties, it is preferable to control elements that are easily oxidized in steel. In the composition of the steel sheet, Si and Mn may be contained in the above ranges, but from the viewpoint of effectively obtaining the effects of suppressing the occurrence of non-plating defects, improving plating peeling resistance (adhesiveness), and preventing the occurrence of uneven appearance, the mass ratio of the Si content to the Mn content (Si/Mn) in the steel is preferably less than 0.1. In order to obtain these effects more effectively, the mass ratio of the Si content to the Mn content (Si/Mn) in the steel is more preferably less than 0.10, still more preferably less than 0.08, and particularly preferably less than 0.06.
The steel of the present invention essentially contains the above-described composition, with the remainder being iron and unavoidable impurities. The above-described component composition may further contain the following components as optional components within a range not to impair the effects of the present invention. When any of the following elements is contained below the lower limit, the optional component is contained as an inevitable impurity. The composition may contain Mg, La, Ce, Bi, W, Pb: 0.002% or less in total as an inevitable impurity.
The above-mentioned component composition may further contain at least 1 of the following (1) to (3) as an optional component in mass%.
(1) 1 or more of Ti, Nb, V and Zr: 0.005 to 0.1 percent in total
(2) 1 or more of Mo, Cr, Cu and Ni: 0.01 to 0.5 percent of the total
(3)B：0.0003％～0.005％
Ti, Nb, V and Zr form carbides and nitrides (also in the case of carbonitrides) with C, N. The fine precipitates contribute to increasing the strength of the steel sheet. In particular, precipitation on soft ferrite increases the strength thereof, and the effect of reducing the strength difference from martensite contributes not only to improvement of bendability but also to improvement of stretch-flange formability. These elements also have an effect of refining the structure of the hot-rolled coil, and contribute to improvement in strength and workability such as bendability by refining the steel structure after subsequent cold rolling and annealing. From the viewpoint of obtaining this effect, it is preferable to contain 1 or more of Ti, Nb, V, and Zr: 0.005% or more in total. However, excessive addition not only increases the deformation resistance during cold rolling and hinders productivity, but also the presence of excessive or coarse precipitates tends to decrease the ductility of ferrite and to decrease the ductility and bendability of the steel sheet. Therefore, 1 or more of Ti, Nb, V, and Zr are preferable: the total content is 0.1% or less.
The elements of Mo, Cr, Cu, and Ni are elements that contribute to strengthening by easily improving hardenability and forming martensite. In order to obtain these effects, the lower limit of 0.01% is defined as a preferable lower limit. Excessive addition of Mo, Cr, Cu and Ni not only saturates the effect and increases the cost, but also Cu induces cracks during hot rolling and causes surface defects. Therefore, 1 or more of Mo, Cr, Cu, and Ni are preferable: the total content is 0.5% or less. Since Ni has an effect of suppressing the occurrence of surface defects due to the addition of Cu, it is preferable to add Ni together with the addition of Cu. Particularly, 1/2 or more is preferable as the Cu content.
B is set to a lower limit necessary for obtaining an effect of suppressing ferrite generation caused during annealing and cooling, and an upper limit is set for the reason that the effect is saturated due to excessive addition thereof. Excessive hardenability also has disadvantages such as cracking of the welded portion during welding. Therefore, the B content is preferably 0.0003% to 0.005%.
The above-mentioned composition may further contain the following components as optional components.
Sb: 0.001% -0.1% and Sn: at least 1 of 0.001% -0.1%
Sb and Sn are effective elements for suppressing the strength reduction of the steel sheet by suppressing decarburization, denitrification, and deboration, and are preferably contained in an amount of 0.001% or more. However, the addition of an excessive amount lowers the surface properties, so that the upper limit thereof is preferably 0.1%.
Ca: 0.0005% or less
When a small amount of Ca is added, the sulfide shape is spheroidized, and the bendability of the steel sheet is improved. On the other hand, if it is excessively added, Ca forms excessive sulfides and oxides in the steel, and deteriorates the workability, particularly the bendability, of the steel sheet, so that the Ca content is preferably 0.0005% or less. The lower limit of the Ca content is not particularly limited, and when Ca is contained, the Ca content is usually 0.0001% or more.
Next, the steel structure of the steel sheet will be explained.
In the steel structure, at least 1 type of inclusion containing Al, Si, Mg and Ca is present in the range from the surface to the 1/3 th position of the plate thickness, the average grain size is 50 μm or less, and the average closest distance of the inclusions is 20 μm or more. The bendability can be improved by adjusting the average particle diameter and the average closest distance of the inclusions to the above ranges and setting the amount of diffusible hydrogen in the steel to a specific range. Note that substances other than at least 1 type of inclusion containing Al, Si, Mg, and Ca were not counted in the measurement of the closest distance of the inclusion.
The average particle size of the inclusions is 50 μm or less, preferably 30 μm or less, and more preferably 20 μm or less. The lower limit is not particularly limited, and is usually 1 μm or more.
The average closest distance of the inclusions is 20 μm or more, preferably 30 μm or more, and more preferably 50 μm or more. The upper limit of the average closest distance of the inclusions is not particularly limited, but is usually 500 μm or less.
The average grain size of inclusions and the average closest distance of inclusions were measured by the methods described in examples.
In the present invention, the steel structure of the steel sheet preferably has 30% to 85% of martensite, 60% or less (including 0%) of ferrite, 15% or less (including 0%) of bainite, and less than 5% or less (including 0%) of retained austenite in terms of area percentage, and the ferrite has an average grain size of 15 μm or less.
Martensite: 30 to 85 percent
Martensite is hard and is effective and necessary for improving the strength of the steel sheet. In order to secure a Tensile Strength (TS) of 1100MPa or more, it is preferably 30% or more in terms of area ratio. From the viewpoint of ensuring the stability of TS, it is preferably 45% or more. The martensite here includes self-tempered martensite obtained by self-tempering during production and optionally tempered martensite obtained by tempering in a subsequent heat treatment. In addition, from the viewpoint of balance between bendability and strength, martensite is preferably 85% or less.
Ferrite: less than 60% (including 0%)
When the steps of heat treatment and plating are performed in an atmosphere in which hydrogen is present, hydrogen enters and remains in the steel. As one method of reducing hydrogen in the steel of the final product as much as possible, ferrite or bainite having a BCC structure is made to appear in the steel structure before the plating is applied. This makes use of the fact that: ferrite or bainite having a BCC structure has a small solid solubility of hydrogen as compared with austenite having an FCC structure. In addition, the soft ferrite improves ductility and bendability of the steel sheet. However, if the ferrite content exceeds 60%, the strength cannot be ensured, so the preferable upper limit is 60%. The ferrite content is usually 2% or more.
The average grain size of ferrite is preferably 15 μm or less. As the ferrite grain size is smaller, the formation and connection of voids on the curved surface can be suppressed, and the bendability can be improved. The average grain size of ferrite is more preferably 10 μm or less, and still more preferably 4 μm or less.
Bainite: less than 15% (including 0%)
Bainite may be contained because it contributes to improvement of bendability, but if it is contained excessively, the required strength cannot be obtained, and therefore 15% or less is preferable. The bainite is at most 2% or more.
Residual austenite less than 5% (including 0%)
Austenite is an fcc phase, and is likely to remain in steel because of its high hydrogen storage capacity and its slow diffusion in steel, as compared with ferrite (bcc phase). In addition, when the retained austenite is transformed into martensite by the work induction, the diffusible hydrogen in the steel may be increased. Therefore, in the present invention, the retained austenite is preferably less than 5%.
In addition, the steel structure may contain precipitates such as pearlite and carbide as structures other than the above-described structure (phase) in the remaining portion, and it is permissible if the total area ratio of these structures from the 1/4-point of the sheet thickness of the steel sheet surface is 10% or less. Preferably 5% or less (including 0%).
The inclusions and area ratios of the steel structure were confirmed by the methods described in examples.
Next, the galvanized layer will be explained. The plating adhesion amount of each surface of the zinc coating is 20-120 g/m2. The adhesive amount is less than 20g/m2In this case, it is difficult to ensure corrosion resistance. Therefore, the amount of adhesion was 20g/m2Above, preferably 25g/m2Above, more preferably 30g/m2The above. On the other hand, if it exceeds 120g/m2The plating peeling resistance is deteriorated. Therefore, the amount of adhesion was 120g/m2Hereinafter, it is preferably 100g/m2Hereinafter, more preferably 80g/m2The following.
The composition of the zinc plating layer is not particularly limited, and may be a general composition. For example, in the case of a hot-dip galvanized layer or an alloyed hot-dip galvanized layer, the following composition is generally used: contains Fe: 20% by mass or less, Al: 0.001 to 1.0 mass%, and further contains 1 or 2 or more species selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi and REM: 0 to 3.5% by mass in total, and the balance of Zn and unavoidable impurities. In the present invention, it is preferable that the plating adhesion amount per one surface is 20 to 120g/m2Hot dip galvanized layer of (2), and further alloyed layerAlloying hot-dip galvanizing layer. In addition, when the plating layer is a hot-dip galvanized layer, the Fe content in the plating layer is preferably less than 7 mass%, and when the plating layer is an alloyed hot-dip galvanized layer, the Fe content in the plating layer is preferably 7 to 20 mass%.
The amount of diffusible hydrogen in the steel measured by the method described in examples of the high-strength galvanized steel sheet according to the present invention is less than 0.25 mass ppm. Diffusible hydrogen in steel deteriorates bendability. If the amount of diffusible hydrogen in the steel is 0.25 mass ppm or more, the bendability is poor even if inclusions or steel structure are appropriately generated.
In the present invention, it is found that a stable improvement effect is obtained by reducing the amount of diffusible hydrogen in steel to less than 0.25 mass ppm. Preferably 0.20 mass ppm or less, more preferably 0.15 mass ppm or less. The lower limit is not particularly limited, and the lower limit is 0 mass ppm because the smaller the amount, the better. In the present invention, before forming and welding the steel sheet, the diffusible hydrogen in the steel must be less than 0.25 mass ppm. When a sample of a product (part) after forming and welding a steel sheet is cut out from the product and the amount of diffusible hydrogen in the steel is measured after the product is left in a normal use environment, if the diffusible hydrogen in the steel is less than 0.25 mass ppm, the amount of diffusible hydrogen before forming and welding may be considered to be less than 0.25 mass ppm.
The high-strength galvanized steel sheet of the present invention has a high Tensile Strength (TS). Specifically, the Tensile Strength (TS) measured by the method described in examples was 1100MPa or more. The thickness of the steel sheet in the high-strength galvanized steel sheet of the present invention is not particularly limited, but is preferably 0.5mm to 3 mm.
Next, a method for producing a high-strength galvanized steel sheet according to the present invention will be described. The manufacturing method of the present invention includes a casting step, a hot rolling step, an acid washing step, a cold rolling step, a pretreatment step, an annealing step, and a plating step. Hereinafter, each step will be explained. The temperature when a slab (steel material), a steel plate, or the like is heated or cooled as described below indicates the surface temperature of the slab (steel material), the steel plate, or the like unless otherwise specified.
The casting step is a step of casting the steel having the above-described composition into a steel material under a condition that a molten steel flow rate at a solidification interface in the vicinity of a meniscus of a mold is 16 cm/sec or more.
Production of a billet (slab (cast sheet))
The steel used in the production method of the present invention is produced by a continuous casting method, generally called slab, and can be produced by an ingot casting method, a thin slab casting method, or the like, for the purpose of preventing macro-segregation of alloy components.
In the case of continuous casting, from the viewpoint of inclusion control, casting is performed under the condition that the flow rate of molten steel at the solidification interface in the vicinity of the meniscus of the mold (hereinafter, also simply referred to as molten steel flow rate) is 16 cm/sec or more. The flow rate of the molten steel is preferably 17 cm/sec or more. Since the steel sheet according to the present invention can be easily obtained by increasing the flow rate of molten steel, the upper limit is not particularly limited, and is preferably 50 cm/sec or less from the viewpoint of handling stability.
The term "the vicinity of the meniscus of the mold" refers to the interface between molten steel and powder used for continuous casting in the mold. In the case of an ingot, it is preferable to sufficiently float impurities during solidification and cut off the portion where the impurities float and gather, and use the portion for the next step.
The hot rolling step is a step of hot rolling the steel slab after the casting step.
In addition to the conventional method of cooling down to room temperature once after the production of a billet and then reheating the billet, a method of hot rolling a warm piece directly in a heating furnace without cooling down to around room temperature, a method of hot rolling immediately after slight additional heating, or a method of hot rolling after keeping a high temperature state after casting can be performed without any problem.
The hot rolling method is not particularly limited, and is preferably performed under the following conditions.
The heating temperature of the billet is preferably in the range of 1100 to 1350 ℃. This is due to: precipitates present in a billet are easily coarsened, and are disadvantageous in the case of securing strength by precipitation strengthening, for example. Or the coarse precipitates may serve as nuclei to adversely affect tissue formation in the subsequent annealing process. Further, it is advantageous to eliminate bubbles, defects, and the like on the slab surface by heating to reduce cracks and irregularities on the steel sheet surface and to realize a smooth steel sheet surface as product quality. From such a viewpoint, the slab heating temperature is defined. In order to obtain such effects, 1100 ℃ or higher is preferable. On the other hand, if it exceeds 1350 ℃, austenite grains coarsen, and the steel structure of the final product also coarsens, which causes a reduction in the strength and bendability of the steel sheet, so that a preferable upper limit is defined.
In the hot rolling step including rough rolling and finish rolling, a slab is generally formed into a thin slab by the rough rolling and into a hot rolled coil by the finish rolling, but there is no problem as long as the slab has a predetermined size, regardless of such a division in accordance with rolling capacity or the like.
As the hot rolling conditions, the following conditions are recommended.
The finish rolling temperature is preferably in the range of 800 to 950 ℃. The temperature is 800 ℃ or higher, so that the structure obtained in the hot-rolled coil is uniform, and the structure of the final product is also uniform. If the tissue is not uniform, the flexibility tends to decrease. On the other hand, if the temperature exceeds 950 ℃, the amount of oxide (scale) formed increases, the interface between the base iron and the oxide becomes rough, and the surface quality after pickling and cold rolling tends to deteriorate. In addition, the grain size becomes coarse, and this tends to cause a decrease in the strength and bendability of the billet and the steel sheet as well.
For refining and uniformizing the structure of the hot-rolled coil (hot-rolled sheet) after the completion of the hot rolling, it is preferable that the cooling is started within 3 seconds after the completion of the finish rolling, the cooling is performed at an average cooling rate of 10 to 250 ℃/s in a temperature range of [ finish rolling temperature ] to [ finish rolling temperature-100 ] ° c, and the coil is wound in a temperature range of 450 to 700 ℃.
The pickling step is a step of pickling the steel sheet after the hot rolling step. The scale was washed off by acid washing. The pickling conditions can be set appropriately.
The cold rolling step is a step of cold rolling the steel sheet after the pickling step at a reduction ratio of 20% to 80%.
The reduction ratio is set to 20% or more in order to obtain a uniform and fine steel structure in the subsequent annealing step. If the content is less than 20%, coarse grains are likely to be formed during annealing, or uneven structures are likely to be formed, and as described above, the strength and workability of the final product sheet may be reduced. Since a high reduction ratio may cause not only a reduction in productivity due to rolling load but also a shape defect, the upper limit is set to 80%. Note that pickling may be performed after cold rolling.
The pretreatment step is a step of heating the steel sheet to a pretreatment heating temperature of 720 ℃ to 880 ℃ after the cold rolling step, cooling the steel sheet from the pretreatment heating temperature to 500 ℃ at an average cooling rate of 2 ℃/sec or more, cooling the steel sheet from 499 ℃ to 200 ℃ at an average cooling rate of 3 ℃/sec or more, and then pickling the steel sheet.
The steel structure having ferrite and martensite is obtained by this pretreatment, and the properties of the final product plate such as strength can be improved. In order to obtain such a microstructure, the pretreatment heating temperature is set to 720 to 880 ℃, but if the pretreatment heating temperature is too high, the following problems arise: there is a tendency that a steel structure or the like for obtaining desired properties is difficult to obtain and the surface properties of a steel sheet are deteriorated, and if it is too low, the effect of improving the properties by the pretreatment is small, and the number of simple steps increases, and the significance of performing the treatment is weak, so that the pretreatment heating temperature is set to 720 to 880 ℃. The pretreatment heating temperature is preferably 720 ℃ to Ac3 ℃.
If the average cooling rate of the pretreatment heating temperature to 500 ℃ is too low, ferrite or pearlite transformation easily occurs in austenite, and therefore the average cooling rate of the pretreatment heating temperature to 500 ℃ is set to 2 ℃/sec or more. The average cooling rate of the pretreatment heating temperature to 500 ℃ is preferably 3 ℃/sec or more. The upper limit of the average cooling rate of the pretreatment heating temperature to 500 ℃ is not particularly limited, but is preferably 200 ℃/s or less from the viewpoint of energy saving of the cooling equipment.
By setting the average cooling rate at 499-200 ℃ to 3 ℃/sec or more, martensite transformation can be reliably caused, and bainite transformation of austenite can be suppressed. The average cooling rate at 499-200 ℃ is preferably 4 ℃/sec or more. The upper limit of the average cooling rate of 499 to 200 ℃ is not particularly limited, but is preferably 200 ℃/s or less from the viewpoint of energy saving of the cooling equipment.
The annealing step is a step of heating the steel sheet after the pretreatment step at an annealing temperature of 740 ℃ to (Ac3+20) ° C while setting the hydrogen concentration in the furnace atmosphere in a temperature range of 500 ℃ or higher to 12 vol% or lower in a continuous annealing line, and then cooling the steel sheet from the annealing temperature to 600 ℃ at an average cooling rate of 3 ℃/sec or higher. The cooling stop temperature of the cooling is not particularly limited. Note that the Ac3 transformation point (also abbreviated as Ac3 in this specification) was calculated as follows.
Ac3(℃)＝910－203(C)1/2+44.7Si－30Mn－11P+700S+400Al
+400Ti。
The symbol of an element in the above formula means that the content (% by mass) of each element is 0.
If the hydrogen concentration in the furnace atmosphere in the temperature range of 500 ℃ or higher is too high, there is a problem that the amount of diffusible hydrogen in the steel specified in the present invention exceeds the upper limit, and therefore the hydrogen concentration in the furnace atmosphere in the temperature range of 500 ℃ or higher is made to be more than 0 vol% and 12 vol% or less. The hydrogen concentration is preferably 10 vol% or less. From the viewpoint of improving the plating property, the hydrogen concentration is preferably 1 vol% or more, and more preferably 3 vol% or more.
If the annealing temperature is too high, there is a problem that the amount of diffusible hydrogen in the steel specified in the present invention exceeds the upper limit, and if it is too low, there is a problem that the tensile strength specified in the present invention cannot be obtained, so the annealing temperature is set to 740 ℃ to (Ac3+20) ° C.
If the average cooling rate of the annealing temperature to 600 ℃ is too low, the amount of martensite for obtaining the desired characteristics cannot be secured, and therefore the average cooling rate is set to 3 ℃/sec or more. The average cooling rate of the annealing temperature to 600 ℃ is preferably 4 ℃/sec or more. The reason for focusing attention on the temperature range of the annealing temperature to 600 ℃ is: this temperature region is a temperature region that affects the amount of austenite that becomes martensite. The upper limit of the average cooling rate of the annealing temperature to 600 ℃ is not particularly limited, but is preferably 200 ℃/s or less from the viewpoint of energy saving of the cooling equipment.
The steel sheet can be cooled from the annealing temperature to 600 ℃ and then temporarily cooled to a temperature lower than the annealing temperature, and then reheated to retain the steel sheet in a temperature range of 450 to 550 ℃. At this time, martensite may be formed when the steel sheet is cooled to the Ms point or less, and then tempered.
The plating step is a step of performing plating treatment on the steel sheet after the annealing step, and cooling the steel sheet at an average cooling rate of 3 ℃/sec or more in a temperature range of 450 to 250 ℃.
If the average cooling rate in the temperature range of 450 to 250 ℃ after the plating treatment is too low, martensite is difficult to be generated in an amount necessary for obtaining the effects of the present invention, and therefore the average cooling rate is set to 3 ℃/sec or more. The average cooling rate of 450 to 250 ℃ after the plating treatment is preferably 5 ℃/sec or more. The reason for focusing on the temperature range of 450 to 250 ℃ is to consider the martensite start temperature (Ms point) from the plating and/or plating alloying temperature. The upper limit of the average cooling rate of 450 to 250 ℃ after the plating treatment is not particularly limited, but is preferably 2000 ℃/s or less from the viewpoint of energy saving of the cooling equipment.
Galvanizing is performed, for example, by immersion in a hot dip galvanizing bath. The hot-dip galvanizing treatment may be performed by a conventional method, and the amount of plating deposit per surface may be adjusted to the above range.
After the galvanization, the alloying treatment for galvanization may be performed immediately as necessary. In this case, the temperature can be maintained at 480 to 580 ℃ for about 1 to 60 seconds.
Next, preferred production conditions in the pretreatment step and the annealing step will be described.
In the pretreatment heating, oxides such as Si and Mn are externally oxidized on the surface layer of the steel sheet to inhibit the plating property, and therefore, pickling is required before annealing. Further, as the amount of external oxidation during the pretreatment heating is increased, the external oxidation in the continuous annealing line can be suppressed, and thus a good plating appearance can be obtained. Therefore, it has been considered that the higher the dew point and the maximum reached temperature of the pre-heat treatment temperature are, the lower the dew point of the continuous annealing line in the annealing step is, and the more favorable plating appearance is easily obtained. For this reason, the best conditions for obtaining a good plating appearance have been intensively studied, and as a result, it has been found that: preferably, the pre-heat treatment and the continuous annealing line are controlled to have different dew points depending on the maximum reaching temperature of the pre-treatment heating. Specific dew point control conditions are as follows.
Preferably, in the pretreatment step, the dew point of an atmosphere of 600 ℃ to the pretreatment heating temperature is Y ℃ or higher when the continuous annealing line is heated at the pretreatment heating temperature, and in the annealing step, the dew point of a temperature region of 700 ℃ or higher in the continuous annealing line is Z ℃ or lower when the continuous annealing line is heated at the annealing temperature, and the pretreatment heating temperature is X ℃, the X, Y and Z satisfy the following relational expressions (i), (ii) or (iii).
(i) X is more than or equal to 720 and less than or equal to 800, and Y-Z is more than or equal to-5
(ii) X is more than 800 and less than or equal to 840, and Y-Z is more than or equal to 0
(iii) X is more than 840 and less than or equal to 880, and Y-Z is more than or equal to 5
The dew point Y ℃ is preferably from-50 ℃ to-25 ℃. When the dew point Y ℃ is lower than-50 ℃, the amount of external oxidation of Si, Mn, etc. at the time of the pre-heat treatment becomes insufficient, resulting in poor plating appearance and plating adhesion. When the dew point Y ℃ exceeds-25 ℃, the adhesion due to Si and Mn oxides is liable to occur, and the operation becomes difficult. The dew point Z ℃ of the continuous annealing line is not particularly limited, and when the dew point Z ℃ is lower than-55 ℃, it is difficult to ensure the airtightness of the furnace body, and the cost is likely to increase. On the other hand, when the dew point Z ° exceeds-30 ℃, external oxidation after continuous annealing tends to be excessive, which may result in poor plating appearance and plating adhesion. Therefore, the dew point Z of the continuous annealing line is preferably-55 ℃ to-30 ℃.
From the viewpoint of reducing the amount of diffusible hydrogen, it is preferable to further include a post-treatment step of heating in an atmosphere having a hydrogen concentration of 5 vol% or less and a dew point of 50 ℃ or less for 30 seconds or more in a temperature range of 50 to 400 ℃ after the annealing step or after the plating step. The post-treatment step is preferably performed as a step subsequent to the annealing step or the plating step.
If the hydrogen concentration or the dew point in the post-treatment step is too high, hydrogen is liable to intrude into the steel on the contrary, and the amount of diffusible hydrogen in the steel specified in the present invention may exceed the upper limit, so that an atmosphere having a hydrogen concentration of 5 vol% or less and a dew point of 50 ℃ or less is preferable.
If the heating time in the temperature range of 50 to 400 ℃ is short, the effect of reducing the amount of diffusible hydrogen in the steel is small, and this step is a simple step increase, so the heating time in the temperature range of 50 to 400 ℃ is preferably 30 seconds or more. The reason for focusing attention on the temperature range of 50 to 400 ℃ is that the dehydrogenation reaction proceeds more than the hydrogen penetration in this temperature range, and the properties of the material and the plating layer may be deteriorated at the temperature or higher.
After the plating step, a width trimming step of performing width trimming may be further provided. In the width trimming step, the end portions of the steel sheet in the sheet width direction are sheared. This not only adjusts the product width, but also removes the diffusible hydrogen from the sheared edge faces, and has the effect of reducing the amount of diffusible hydrogen in the steel.
The production of the high-strength galvanized steel sheet of the present invention may be carried out in a continuous annealing line or off-line.
< high strength component and method for manufacturing the same >
The high-strength member of the present invention is obtained by at least one of forming and welding the high-strength galvanized steel sheet of the present invention. The method for manufacturing a high-strength member of the present invention includes a step of performing at least one of forming and welding on the high-strength galvanized steel sheet manufactured by the method for manufacturing a high-strength galvanized steel sheet of the present invention.
The high-strength member of the present invention has excellent bendability, and therefore can suppress cracking after bending, and has high reliability as a structural surface of the member. In addition, the high-strength member is excellent in plating properties, particularly plating peel resistance. Therefore, for example, when a steel sheet is press-molded to form a part, adhesion of zinc powder or the like to a press die due to zinc plating delamination can be suppressed, and occurrence of surface defects in the steel sheet due to the adhesion can be suppressed. Therefore, the productivity in press molding is high.
The molding may be performed by any general processing method such as press processing without limitation. Further, general welding such as spot welding and arc welding can be used without limitation. The high-strength member of the present invention is applicable to, for example, automobile parts.
Examples
[ example 1]
In order to confirm the influence of the amount of hydrogen in the steel, the study shown in example 1 was performed.
Molten steel having a composition shown in Table 1 was melted in a converter, and slabs were produced at an average molten steel flow rate of 18 cm/sec at a solidification interface in the vicinity of a meniscus of a mold and an average casting speed of 1.8 m/min. The slab was heated to 1200 ℃ and hot rolled at a finish rolling temperature of 840 ℃ and a winding temperature of 550 ℃. The hot-rolled steel sheet obtained from the hot-rolled coil was pickled and then made into a cold-rolled steel sheet having a thickness of 1.4mm at a cold reduction of 50%. The cold-rolled steel sheet was heated to 790 ℃ (within the range of Ac3 point +20 ℃ or lower) by annealing treatment in an annealing furnace atmosphere having various hydrogen concentrations and a dew point of-30 ℃, cooled to 520 ℃ at an average cooling rate of 3 ℃/sec to 600 ℃, left for 50 seconds, and then subjected to galvanizing alloying treatment to be cooled from 450 ℃ to 250 ℃ at an average cooling rate of 6 ℃/sec, thereby producing a high-strength galvannealed steel sheet (product sheet).
Samples were cut from each steel sheet, and hydrogen analysis (amount of diffusible hydrogen) and bendability in the steel were evaluated. The results are shown in FIG. 1.
Amount of Hydrogen in Steel (diffusible Hydrogen amount)
The amount of hydrogen in the steel was measured by the following method. First, a test piece of about 5X 30mm was cut out from a plated steel sheet, plated on the surface of the test piece was removed by a router (precision grinder), and the test piece was placed in a quartz tube. Then, the quartz tube was replaced with Ar, and then the temperature was raised at 200 ℃/hr, and hydrogen generated until 400 ℃ was measured by a gas chromatograph. Thus, the amount of hydrogen released was measured by a temperature rise analysis method. The cumulative value of the hydrogen amounts detected in the temperature region from room temperature (25 ℃) to less than 210 ℃ was taken as the diffusible hydrogen amount in the steel.
Flexibility
A long test piece of 25X 100mm was cut out from the produced plated steel sheet so as to have a short side in a direction parallel to the rolling direction. Next, a 90 ° V bending test was performed so that the rolling direction was a ridge line at the time of bending. The stroke speed was set to 50mm/min, and pressing was performed by pressing the die with a load of 10 tons for 5 seconds. The test was carried out by varying the tip R of the V-shaped punch in 0.5 steps, and the vicinity of the ridge line of the test piece was observed with a lens at 20 magnifications to confirm the presence or absence of cracks. R/t was calculated from the smallest R that did not cause cracks and the sheet thickness (tmm, using the value of rounding the third decimal place to the second decimal place) of the test piece, and was used as an index of bendability. The smaller the R/t value, the better the bendability.
When the diffusible hydrogen content in the steel is less than 0.25 mass ppm, the steel is excellent in stabilized bendability (R/t). The conditions for inclusion in the excellent sample are within the range of the present invention.
Figure BDA0002706329640000181
[ example 2]
In example 2, the following galvanized steel sheets were produced and evaluated.
Molten steel having a composition shown in table 2 was melted in a converter, and a slab cast under the conditions shown in table 3 was reheated to 1200 ℃, hot-rolled at a finish rolling temperature of 800 to 830 ℃, and a hot-rolled coil was produced at a winding temperature of 560 ℃. The hot-rolled steel sheet obtained from the hot-rolled coil was subjected to acid pickling, and the steel sheet was subjected to the steps of cold rolling, pretreatment, annealing, plating treatment, width dressing, and post-treatment under the conditions shown in table 3, to thereby produce a galvanized steel sheet having a thickness of 1.4 mm. Immediately after the plating treatment (galvanizing treatment), alloying treatment for galvanizing was performed at 500 ℃ for 20 seconds. The width trimming and post-treatment steps are performed under only a part of the manufacturing conditions.
Through the above steps, samples were collected from the obtained plated steel sheets, and structure observation and tensile test were performed by the following methods, and Tensile Strength (TS), amount of hydrogen in steel (diffusible hydrogen amount), bendability, and percentage of steel structure were evaluated and measured. In addition, the plating property was evaluated. The evaluation method is as follows.
(1) Tensile test
Tensile test pieces (JIS Z2201) No. 5 were collected from the plated steel sheet in a direction perpendicular to the rolling direction, and a tensile test was conducted at a constant tensile rate (crosshead speed) of 10 mm/min. The tensile strength is a value obtained by dividing the maximum load in the tensile test by the cross-sectional area of the test piece parallel portion at the initial stage. The thickness of the plate in the calculation of the cross-sectional area of the parallel portion is a plate thickness value including the plating thickness.
(2) Hydrogen quantity in Steel (diffusible hydrogen quantity)
The procedure was carried out in the same manner as in example 1.
(3) Flexibility
The procedure was carried out in the same manner as in example 1. In the present evaluation, R/t.ltoreq.3.5 was evaluated as excellent in bendability.
(4) Tissue observation
A test piece for texture observation was collected from the produced hot-dip galvanized steel sheet, and after polishing the L section (a sheet thickness section parallel to the rolling direction), the L section was corroded with a nital solution, and an image obtained by 3 fields or more of observation by SEM at a magnification of 1500 times was analyzed (the area ratio was measured in accordance with the field of observation, and the average value was calculated). The observation position is a position near the sheet thickness 1/4 from the sheet thickness surface. However, the volume fraction (volume fraction is regarded as area fraction) of the retained austenite is quantified by the X-ray diffraction intensity, and therefore the total of the respective structures may exceed 100%. In table 4, F denotes ferrite, M denotes martensite (including tempered martensite), B denotes bainite, and γ denotes retained austenite. The average grain size of ferrite was determined by observing 10 particles with SEM, determining the area fraction of each particle, calculating the circle equivalent diameter, and averaging the circle equivalent diameters.
In the above structure observation, in some examples, aggregation of pearlite, precipitates, and inclusions as other phases is observed.
(5) Observation of inclusions
The ridge line portion of the test piece subjected to the 90 ° V bending test was forcibly fractured, and the cross section of the steel sheet was observed by SEM. With respect to inclusions present on the surface layer of the test piece, that is, from the outer surface of the bend to the position 1/3 in the sheet thickness, the composition was confirmed by EDX qualitative analysis, and after identifying oxides containing at least 1 or more of Al, Si, Mg, and Ca, the longest diameter (the size of the portion having the longest particle width) of the inclusions in the image was measured, and the longest diameter was regarded as the particle diameter, and the average particle diameter was determined. In this field of view, the distance (closest distance) to the closest inclusion is determined for any inclusion present in the range from the surface to the position 1/3 in the sheet thickness, and the average closest distance is determined by averaging the distances for all the inclusions.
(6) Plating property
The surface properties (appearance) of the produced hot-dip galvanized steel sheet were visually observed to analyze the presence or absence of a defect of no plating. The non-plating defect is on the order of several μm to several mm, and means a region where the steel sheet is exposed without plating.
The produced hot-dip galvanized steel sheet was analyzed for plating peeling resistance (adhesion). In this example, the amount of the peeled off material on the transparent adhesive tape was determined by an X-ray fluorescence method as a Zn count by transferring the peeled off material to the transparent adhesive tape by pressing the transparent adhesive tape on the processed portion of the hot-dip galvanized steel sheet bent at 90 °. The measurement conditions were: mask diameter 30mm, acceleration voltage of X-ray fluorescence 50kV, acceleration current 50mA, and measurement time 20 seconds.
The plating property was evaluated according to the following criteria. The results are shown in Table 4. In the present invention, the following grade A, B or C having no unplated defects at all was regarded as acceptable.
A: no plating defects at all, and a Zn count of less than 7000.
B: no non-plating defect at all, and a Zn count of 7000 or more and less than 8000.
C: no non-plating defect at all, and a Zn count of 8000 or more.
D: non-plating defects are generated.
Figure BDA0002706329640000211
Figure BDA0002706329640000221
Figure BDA0002706329640000231
The galvanized steel sheet of the present invention example obtained from the components and production conditions within the range of the present invention has a TS of not less than 1100MPa, a high strength, an R/t of not more than 3.5, excellent bendability, and excellent plating properties.
[ example 3]
In example 3, the following galvanized steel sheets were produced and evaluated. In the production method of example 3, the preferable production method of the galvanized steel sheet of the present invention was examined in more detail by controlling the dew point in the pre-heat treatment and the continuous annealing line.
Molten steel having a composition shown in table 5 was melted in a converter, and a slab cast under the conditions shown in table 6 was reheated to 1200 ℃, hot-rolled at a finish rolling temperature of 800 to 830 ℃, and a hot-rolled coil was produced at a winding temperature of 560 ℃. A hot-rolled steel sheet obtained from the hot-rolled coil was subjected to acid pickling, and the steel sheet was subjected to the steps of cold rolling, pretreatment, annealing, plating treatment, width dressing, and post-treatment under the conditions shown in table 6, thereby producing galvanized steel sheets having a thickness of 1.4 mm. Immediately after the plating treatment (galvanizing treatment), alloying treatment for galvanizing was performed at 500 ℃ for 20 seconds. The width trimming and post-treatment steps are performed under only a part of the manufacturing conditions.
Through the above steps, samples were collected from the obtained plated steel sheets, and the Tensile Strength (TS), the amount of hydrogen in steel (diffusible hydrogen amount), the bendability, and the percentage of the steel structure were evaluated and measured by performing structure observation and tensile test in the same manner as in example 2. In addition, the plating property was evaluated by the same method as in example 2.
Further, except that the alloying treatment for galvanizing was not performed, galvanized steel sheets were also produced under the same production conditions as production conditions 3 to 7 in table 6. Then, the surface properties (appearance) of the galvanized steel sheet were visually observed, and the presence or absence of plating and the area exposed to the steel sheet (presence or absence of plating defects) were analyzed on the order of several μm to several mm. As a result of the analysis, it was confirmed that the galvanized steel sheet had no unplated defects and had good plating properties.
Figure BDA0002706329640000251
Figure BDA0002706329640000261
Figure BDA0002706329640000271
The galvanized steel sheet of the present invention example obtained from the components and production conditions within the range of the present invention has a TS of not less than 1100MPa, a high strength, an R/t of not more than 3.5, excellent bendability, and excellent plating properties. In addition, it is known that control of the dew point in the pretreatment step and the annealing step is important in order to improve the plating property.
[ example 4]
The zinciferous coated steel sheets of No.3-2 (inventive example) in Table 6 of example 3 were press-formed to manufacture the parts of the inventive example. Furthermore, the galvanized steel sheets of No.3-2 (inventive example) in Table 6 of example 3 and the galvanized steel sheets of No.3-3 (inventive example) in Table 6 were joined by spot welding to manufacture the members of the inventive examples. It was confirmed that the members of the examples of the present invention are excellent in bendability and plating property, and thus can be suitably used for automobile members and the like.
Industrial applicability
The high-strength galvanized steel sheet of the invention not only has high tensile strength, but also has good performanceFlexibility and plating ability. Therefore, when the high-strength galvanized steel sheet of the present invention is used for a frame member of an automobile body, particularly, a vehicle body periphery which affects collision safety, the high-strength galvanized steel sheet can improve safety performance, and contribute to weight reduction of the automobile body by a high-strength thin-walled effect, thereby contributing to CO2Discharge, etc. Further, since the coating composition has both good surface properties and good plating quality, it can be actively used in a portion where rain and snow corrosion is concerned, such as a traveling system, and can be expected to improve the performance of rust prevention and corrosion resistance of a vehicle body. Such properties are effective not only for automobile parts but also in the civil engineering, construction and household electrical appliance fields.

Claims (12)

1. A high-strength galvanized steel sheet comprising a steel sheet and a galvanized layer on the surface of the steel sheet,
the steel sheet has a composition and a steel structure,
the composition is characterized in that the steel composition contains C: 0.08-0.20%, Si: less than 2.0%, Mn: 1.5% -3.5%, P: 0.02% or less, S: 0.002% or less, Al: 0.10% or less and N: less than 0.006%, the remainder being Fe and unavoidable impurities,
in the steel structure, the average grain diameter of at least 1 type of inclusion containing Al, Si, Mg and Ca existing in the range from the surface to the 1/3 th position of the plate thickness is below 50 μm, the average closest distance of the inclusion is above 20 μm,
the plating adhesion amount of each surface of the galvanized layer is 20g/m2～120g/m2，
The amount of diffusible hydrogen contained in the steel is less than 0.25 mass ppm,
the tensile strength of the high-strength galvanized steel sheet is 1100MPa or more.
2. The high-strength galvanized steel sheet according to claim 1, wherein a mass ratio of the Si content to the Mn content in the steel, Si/Mn, is less than 0.1.
3. The high-strength galvanized steel sheet according to claim 1 or 2, wherein the composition further contains at least 1 of the following (1) to (3) in mass%,
(1) 1 or more of Ti, Nb, V and Zr: 0.005 to 0.1 percent in total
(2) 1 or more of Mo, Cr, Cu and Ni: 0.01 to 0.5 percent of the total
(3)B：0.0003％～0.005％。
4. The high-strength galvanized steel sheet according to any one of claims 1 to 3, wherein the composition further contains, in mass%, Sb: 0.001% -0.1% and Sn: 0.001-0.1% of at least 1.
5. The high-strength galvanized steel sheet according to any one of claims 1 to 4, wherein the composition further contains, in mass%, Ca: less than 0.0005%.
6. The high-strength galvanized steel sheet according to any one of claims 1 to 5, wherein the steel structure has 30 to 85% of martensite, 60% or less and including 0% of ferrite, 15% or less and including 0% of bainite, and less than 5% and including 0% of retained austenite in terms of area ratio,
the ferrite has an average grain diameter of 15 μm or less.
7. A method for producing a high-strength galvanized steel sheet, comprising the steps of:
a casting step of casting a steel having the composition according to any one of claims 1 to 5 under a condition that a molten steel flow velocity at a solidification interface in the vicinity of a meniscus of a mold is 16 cm/sec or more to produce a steel material;
a hot rolling step of hot rolling the steel slab after the casting step;
a pickling step of pickling the steel sheet after the hot rolling step;
a cold rolling step of cold rolling the steel sheet after the pickling step at a reduction ratio of 20% to 80%;
a pretreatment step of heating the steel sheet after the cold rolling step to a pretreatment heating temperature of 720 ℃ to 880 ℃, thereafter cooling the steel sheet from the pretreatment heating temperature to 500 ℃ at an average cooling rate of 2 ℃/sec or more, cooling the steel sheet from 499 ℃ to 200 ℃ at an average cooling rate of 3 ℃/sec or more, and then pickling the steel sheet;
an annealing step of heating the steel sheet after the pretreatment step at an annealing temperature of 740 ℃ to (Ac3+20) ° C while setting the hydrogen concentration in the furnace atmosphere in a temperature range of 500 ℃ or higher to 12 vol% or lower using a continuous annealing line, and then cooling the steel sheet from the annealing temperature to 600 ℃ at an average cooling rate of 3 ℃/sec or higher; and
a plating step of plating the steel sheet after the annealing step, and then cooling the steel sheet in a temperature range of 450 to 250 ℃ at an average cooling rate of 3 ℃/sec or more,
in the pretreatment step, when the heating is performed at the pretreatment heating temperature, the dew point of the atmosphere at 600 ℃ to the pretreatment heating temperature is Y ℃ or higher,
in the annealing step, when heating is performed at the annealing temperature, the dew point of a temperature region of 700 ℃ or higher in the continuous annealing line is Z ℃ or lower, and
when the pretreatment heating temperature is set to X ℃,
said X, Y and Z satisfying the following relations (i), (ii) or (iii),
(i) x is more than or equal to 720 and less than or equal to 800, and Y-Z is more than or equal to-5
(ii) X is more than 800 and less than or equal to 840, and Y-Z is more than or equal to 0
(iii) X is more than 840 and less than or equal to 880, and Y-Z is more than or equal to 5.
8. The method for manufacturing a high-strength galvanized steel sheet according to claim 7, further comprising a width trimming step of performing width trimming after the plating step.
9. The method for producing a high-strength galvanized steel sheet according to claim 7 or 8, further comprising a post-treatment step of heating in an atmosphere having a hydrogen concentration of 5 vol% or less and a dew point of 50 ℃ or less for 30 seconds or more in a temperature range of 50 to 400 ℃ after the annealing step or after the plating step.
10. The method for producing a high-strength galvanized steel sheet according to any one of claims 7 to 9, wherein in the plating step, an alloying treatment is performed immediately after the plating treatment.
11. A high-strength member obtained by at least one of forming and welding the high-strength galvanized steel sheet according to any one of claims 1 to 6.
12. A method for producing a high-strength member, comprising a step of performing at least one of forming and welding on a high-strength galvanized steel sheet produced by the method for producing a high-strength galvanized steel sheet according to any one of claims 7 to 10.
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